Acid alpha-glucosidase (GAA) is a lysosomal enzyme that hydrolyzes the alpha 1-4 linkage in maltose and other linear oligosaccharides, including the outer branches of glycogen, thereby breaking down excess glycogen in the lysosome (Hirschhorn et al. (2001) in The Metabolic and Molecular Basis of Inherited Disease, Scriver, et al., eds. (2001), McGraw-Hill: New York, p. 3389-3420). Like other mammalian lysosomal enzymes, GAA is synthesized in the cytosol and traverses the ER where it is glycosylated with N-linked, high mannose type carbohydrate. In the golgi, the high mannose carbohydrate is modified on lysosomal proteins by the addition of mannose-6-phosphate (M6P) which targets these proteins to the lysosome. The M6P-modified proteins are delivered to the lysosome via interaction with either of two M6P receptors. The most favorable form of modification is when two M6Ps are added to a high mannose carbohydrate.
Insufficient GAA activity in the lysosome results in Pompe disease, a disease also known as acid maltase deficiency (AMD), glycogen storage disease type II (GSDII), glycogenosis type II, or GAA deficiency. The diminished enzymatic activity occurs due to a variety of missense and nonsense mutations in the gene encoding GAA. Consequently, glycogen accumulates in the lysosomes of all cells in patients with Pompe disease. In particular, glycogen accumulation is most pronounced in lysosomes of cardiac and skeletal muscle, liver, and other tissues. Accumulated glycogen ultimately impairs muscle function. In the most severe form of Pompe disease, death occurs before two years of age due to cardio-respiratory failure.
Presently, there is no approved treatment available to cure or slow the progress of Pompe disease. Enzyme replacement therapeutics currently in clinical trials require that administered recombinant GAA be taken up by the cells in muscle and liver tissues and be transported to the lysosomes in those cells in an M6P-dependent fashion. However, recombinant GAA produced in engineered CHO cells and in the milk of transgenic rabbits, two sources of enzymes used in recent Pompe enzyme replacement therapy trials, contains extremely little M6P (Van Hove et al. (1996) Proc Natl Acad Sci USA, 93(1):65-70; and U.S. Pat. No. 6,537,785). Therefore, M6P-dependent delivery of recombinant GAA to lysosomes is not efficient, requiring high dosages and frequent infusions. Accordingly, there remains a need for new, simpler, more efficient, and more cost-effective methods for targeting therapeutic GAA enzymes to patient lysosmoes.